1. Field of Use
This invention relates to an improved apparatus fir measuring relative humidity. More specifically, the invention relates to a high precision electronic. Psychrometer operable at low temperatures and high humidity environments.
2. Description of Prior Art (Background)
In general a psychrometer is an instrument consisting of two thermometers which are used in the measurement of the moisture content, or relative humidity (RH) of air or other gases. The bulb or sensing area of one of the thermometers either is covered by a thin piece of clean muslin cloth, or other wick material, wetted uniformly with distilled water or is otherwise coated with a film of distilled water. The temperatures of both the bulb and the air contacting the bulb are lowered by the evaporation which takes place when unsaturated air moves past the wetted bulb. An equilibrium temperature, termed the wet-bulb temperature will be reached; the equilibrium temperature closely approaches the lowest temperature to which air can be cooled by the evaporation of water into the unsaturated air. Moisture parameters, such as relative humidity and dew-point temperature, can be evaluated from the wet- and dry-bulb measurements by means of psychrometric tables and generally accepted closed form formulae for calculating water/air mixtures.
Relative Humidity (RH) is a measure of the degree to which air is saturated with water compared to the highest level of saturation at a given temperature. This is a ratio of the partial pressure (proportional content) of water in air at the actual conditions to the partial pressure of water in air at saturation (100% RH). Partial pressures of water in air are related to temperature.
The traditional method for determining RH is to use a manual sling Psychrometer which has two thermometers, one with a dry bulb and one with a wet bulb. The dry bulb thermometer is typical of thermometers in use in other applications and simply measures the air temperature. The wet bulb thermometer has a water saturated wick around it. When the thermometer is swung in the air to move air over the wet bulb, evaporation of water from this wick depresses the temperature of the bulb to a degree that corresponds to the saturation partial pressure of water in the air at the dry bulb temperature. Comparison of these two temperatures can provide an indirect measure of RH.
However, the long-term (6-12 month) storage of crops requires control of both storage temperature and humidity. Storage temperature is depressed to 32-40 degrees F. (crop dependent) in order to minimize the rate of respiration in the crops. Humidity is generally raised to 80-98% RH to reduce desiccation yet still avoid liquid water condensation on the crops. In recent field research pertaining to improved crop storage methods, it has been determined that there is a lack of suitable equipment for humidity measurement and control at low storage temperatures and high humidity.
The vast majority of humidity sensing equipment available is based on moisture absorbing, materials whose capacitance changes depending on the material moisture content. These sensors tend to have a precision of +/−2% RH from 20-80% RH at 70 degrees F., but then lose precision in the higher RH range and lower temperature range, straying, to +/−5% RH. It is this range that is most needed by those storing winter crops. Some sensors exist which demonstrate +/−2% RH precision up to 98% RH. But in all of these sensor types, excursions to 100% RH results in reduced precision and accuracy and can cause a mechanical failure or a need for recovery (heat and dry) in order to reuse the sensor. Additionally, these sensors may also suffer an unrecoverable electronic failure.
Thus, there is a technical challenge which exists in the measurement of high humidity in low temperature conditions; and, therefore control of equipment (e.g., humidifiers, dehumidifiers) based on these measurements.